Paul M.Sutter (opens in new tab) is an astrophysicist State University of New York (opens in new tab) Stony Brook and Flatiron Institute,”ask an astronaut (opens in new tab)“ and”space radio (opens in new tab)“and” author ofhow Ton (opens in new tab)o Die in space. “
Astronomers hope to use pulsars scattered around the Milky Way as giant gravitational wave detectors. But why do we need them and how do they work?
Gravitational waves, or ripples in the fabric of spacetime, from a variety of sources, jiggling constantly throughout the universe. Now, you are being stretched and squeezed slightly as the waves pass through you.those waves come from the merger black holethe explosion of giant stars, and even the very first moments of the universe big Bang.
superior Earth, we have developed extremely sensitive gravitational wave detectors that can sense brief but loud events, such as black hole mergers, which last only a few seconds but produce signals so huge that we can detect them. (“Huge” is a relative term here; the distortion caused by the passing wave is smaller than the width of the nucleus.)
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But ground-based detectors have a much harder time finding low-frequency gravitational waves, because they take weeks, months, or even years to travel through Earth. These low-frequency waves come from the mergers of giant black holes, which take much longer to merge than their smaller counterparts. Our detectors simply do not have the sensitivity to measure these tiny differences over such a long time span. For this, we need a much larger detector.
So instead of using instruments on the ground, we can use distant pulsars to help us measure gravitational waves. That’s the idea behind the so-called pulsar timing array.
Start the pulsar
Pulsar Already fantastic objects, especially for pulsars used as gravitational wave detectors.
Pulsars are the remaining cores of giant stars Star And is one of the strangest objects known to inhabit the universe.They are ultra-dense spheres made almost entirely of neutrons, some of which electronic And protons put in a good measure. These spinning charges create incredibly strong magnetic fields—in some cases, the most powerful in the universe.
Those strong magnetic fields also excite strong electric fields.Together they power the radiation beam (if you get death star Vibration here, you’re not far) explode from the magnetic poles in every direction. These magnetic poles don’t always align with the pulsar’s axis of rotation, just as Earth’s north and south magnetic poles don’t align with Earth’s axis of rotation.
This forces the radiation beam to sweep across a circle in the sky. As these beams pass through Earth, we see them as periodic flashes of radio emissions, placing the “pulses” in a “pulsar.”
related: Gravitational waves play with fast-spinning stars, study shows
Pulsars are very regular. They are so heavy and spin so fast that we can use their flashes as extremely accurate clocks.But most pulsars are vulnerable to random Starquake (When the contents of the star move around, interfering with the pulsar’s rotation), glitches and slowdowns that change its regularity. This means that most pulsars are not suitable for studying gravitational waves.
So the timing array relies on a subset of pulsars called millisecond pulsars, which, as the name suggests, have a rotation period of a few milliseconds. Astronomers think of millisecond pulsars as “resurrected” pulsars that spin at incredible speeds after falling material from a companion star accelerates them, like an adult pushing a child on a school carousel.
Because of their ludicrous speed, millisecond pulsars can maintain astonishing precision over very long timescales. For example, a pulsar PSR B1937+21 has a rotation period of 1.5578064688197945 +/- 0.0000000000000004 seconds.This is the same level of precision as ours best atomic clock.
Those millisecond pulsars are perfect gravitational wave detectors.
Here’s how it works. First, astronomers observe as many millisecond pulsar rotation periods as possible. If a gravitational wave passes between Earth, a pulsar, or even between us, it will change the distance between Earth and the pulsar as it passes. As the wave moves, the pulsar will appear slightly closer, then slightly further, then slightly closer, and so on, until the wave continues to move.
To us, the change in distance is like a change in the rotation period. One flash of a pulsar might come too early; then another might come too late. For a typical gravitational wave, the change in time is very small—only 10 or 20 nanoseconds every few months. But millisecond pulsar measurements are sensitive enough to detect these changes—at least in principle.
The “array” part of the Pulsar Timing Array comes from studying multiple pulsars at once and looking for correlated motion: if a gravitational wave passes through a region of space, the timings of all pulsars from that direction will move in unison.
For decades, many collaborating institutions around the world have been using radio telescopes to study pulsar timing arrays. So far, they have had limited success, finding temporal variations of various pulsars, but no sign of any correlation. But every year, these techniques get better, and hopefully soon, these arrays will unravel a large part of the gravitational wave universe.
Learn more by listening to the “Ask a Spaceman” podcast on iTunes (opens in new tab) and askaspaceman.com.Ask your own question on Twitter using #AskASpaceman or follow Paul @PaulMattSutter and facebook.com/PaulMattSutter.
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